Biomedical subjects

L H Carney

Publications and source records attributed to L H Carney.

26 records · Page 2Linked to original sources

PubMed1990

Interaural time sensitivity in the inferior colliculus of the albino cat.

Anatomical studies of the Creel albino cat have demonstrated a pronounced atrophy of cells in the medial superior olive, a structure thought to be important for the detection of interaural time differences (ITDs). We looked for physiological abnormalities in the binaural interaction of cells in three albino cats by recording from single cells in the central nucleus of the inferior colliculus to ITDs of tones and noise. We found that the sensitivity to ITDs of tones and noise was somewhat diminished in the albino cats as compared to normally pigmented cats, though this deficit was only evident when a population of cells was examined. The range of sensitivity of individual cells for both tones and noise was the same in albinos and pigmented animals. Our anatomical measurements showed a smaller reduction in cross-sectional area of cells in the medial superior olive than that reported earlier, and the cell bodies in the medial superior olive of the albinos were less elongated than in normal cats.

Albinism↗

PubMed1990

Sensitivities of cells in anteroventral cochlear nucleus of cat to spatiotemporal discharge patterns across primary afferents.

1. This study tested the hypothesis that a cell in the anteroventral cochlear nucleus (AVCN) that receives convergent input from auditory nerve (AN) fibers can be sensitive to the temporal pattern of discharges on the set of AN fibers providing its input. 2. The temporal discharge pattern across the population of low-frequency AN fibers was manipulated by varying the phase spectra of complex stimuli that had fixed, flat magnitude spectra. By introducing a phase shift with variable slope at a particular frequency, the relative times of discharge of phase-locked neurons with different characteristic frequencies (CFs) could be varied. In this manner the overall spatiotemporal discharge pattern across the array of AN fibers was systematically manipulated. 3. Some low-frequency cells in the AVCN were sensitive to changes in the slope of the phase transition of the complex stimulus. The cells that were sensitive came from several different cell types in the AVCN. Their responses were consistent with the hypothesis that these cells were sensitive to the temporal relationships between discharges on their primary inputs and that they received inputs with different CFs, because the phase shifts introduced relative time differences between different frequencies. 4. Other cells were not sensitive to the degree of phase shift of the stimulus. This insensitivity implied either that these cells received inputs of the same, or nearly the same, CF, or that they were not sensitive to the time differences introduced by these changes in the phase spectra, or both. 5. The cells that were sensitive to the manipulations of the phase spectrum were located in the posterior region of anterior AVCN and in the posterior region of AVCN and thus were presumably either globular bushy, small spherical bushy, or stellate cells. No sensitive cells were located in the most anterior region of the AVCN, where large spherical bushy cells are located. 6. Temporal discharge patterns across the AN population in response to complex stimuli change as a function of level. Accordingly, the sensitivity of neurons to changes in the phase transitions of the complex stimuli used in this study was often affected by the level of the stimulus. 7. The sensitivity to changes in the phase spectrum was a frequency-specific effect. That is, a cell was most sensitive to changes made in phase that were centered near its CF and less sensitive to changes in phase that were introduced at frequencies below or above CF.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1989

Responses of low-frequency cells in the inferior colliculus to interaural time differences of clicks: excitatory and inhibitory components.

1. We studied extracellular responses of low-frequency cells in the central nucleus of the inferior colliculus (ICC) to interaural time differences (ITDs) of clicks and compared their responses to ITDs of noise and tones. Most cells that displayed sensitivity to ITDs of clicks responded cyclically as a function of ITD with central peaks and troughs at the same ITDs as in response to noise. The positions of these peaks and troughs also matched those predicted from tonal ITD curves. Thus over the range of physiologically relevant ITDs, the binaural cells in the ICC showed similar sensitivity to ITDs of tones, noise, and clicks. 2. The transient nature of the response to a click allowed association of individual discharges with either the ipsilateral or contralateral stimulus when the binaural stimulus included a large ITD. We studied the influence of the click presented to one side on responses to the click presented to the other side. By examining responses to clicks with large ITDs, ranging from 2 to 3 up to 200 ms, we could identify both excitatory and inhibitory components in response to binaural clicks. 3. For many cells, there was evidence for a short-lasting excitation arising from one or both inputs of the binaural stimulus. Inhibitory interactions could also be demonstrated over a large range of ITDs. Long-lasting, late inhibitory components arose from both contralateral and ipsilateral inputs. In 87% of cells that were driven by the contralateral input, a late inhibitory component originating from the ipsilateral side was detected. In all cells that were driven by the ipsilateral side, a late inhibitory contralateral component was detected. This late inhibition of the excitatory response to one side by a leading stimulus to the other side could be evoked even when the leading stimulus was not effective in evoking an excitatory response. 4. Some cells also exhibited an early inhibitory component that preceded the excitation. An early contralateral inhibition was detected in 44% of cells that were driven by the ipsilateral input, whereas an early ipsilateral component was detected in 17% of cells driven by the contralateral input. 5. We confirmed hypotheses about the laterality and time course of the inhibitory and excitatory components by introducing interaural level differences (ILDs) into the binaural clicks and thus varying the strengths of the different components. 6. Inhibitory components may play a role in shaping the sensitivity of individual cells to ITDs of stimuli other than clicks; they were also apparent in responses to noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1988

The radiation impedance of the external ear of cat: measurements and applications.

The configuration of external ears varies dramatically among mammalian species. In order to relate these structural differences to acoustic performance, it is useful to determine the "output" (radiation) impedance of the external ear. Measurements were made of the radiation impedance ZE of the cat external ear looking out from the location of the tympanic membrane. Freshly excised external ears were coupled to a calibrated sound source at the tympanic ring, and the resulting sound pressure at the source was measured. The ZE calculated from these measurements is masslike at frequencies below 2 kHz and approximately resistive above 4 kHz. The contributions of anatomically distinct sections of the external ear to ZE were assessed by measuring the impedance before and after surgical removal of the pinna flange and of the concha. Mean measurements of the lengths and cross-sectional areas of components of the external ear are used in a simple model that consists of a uniform tube and an exponential horn; the radiation impedance of the model shows many of the features of the measured ZE's. Measurements of the input impedance of the middle ear are combined with ZE to infer the diffuse-field absorption cross section ADF, which is a measure of the ear's performance as a coupler of acoustic power. It is suggested that ADF is useful for across-species comparisons of the performance of external and middle ears.

Acoustics↗

PubMed1988

Temporal coding of resonances by low-frequency auditory nerve fibers: single-fiber responses and a population model.

1. We recorded responses of low-frequency auditory nerve fibers (characteristic frequency (CF) less than 3 kHz) in the cat to resonant stimuli with varied natural frequencies, damping coefficients, and sound pressure levels. Responses to resonances were synchronized to frequencies lying between the peak frequency of the stimulus spectrum and a frequency near the fiber's CF. The frequency of the dominant synchrony in the response varied systematically as a function of the stimulus parameters. 2. More lightly damped resonances, which have sharp spectral peaks, elicited synchrony closer to the peak frequency, whereas the broader peaks of more highly damped resonances elicited synchrony closer to the fiber's CF. Thus as the stimulus was varied from an undamped tone to a highly damped transient, the dominant component of the synchronized response moved from the peak frequency of the stimulus toward the CF of the fiber. The trajectory of the dominant component varied as a function of stimulus level, with higher levels resulting in synchrony biased toward the peak of the stimulus spectrum over a wider range of damping. 3. The frequency tuning and synchronization characteristics of a fiber, along with the stimulus parameters, determined the temporal properties of its response to complex stimuli. Using reverse correlation (revcor) filters to characterize the tuning and synchronization of auditory nerve fibers, we were able to predict the temporal properties of responses to resonant stimuli. 4. A parametric model was fit to measured revcor functions derived from responses of auditory nerve fibers to wideband noise. In this way, a bank of model revcor filters was developed based on our population of measured filters. 5. The filter bank was used to model the response of a population of auditory nerve fibers to resonances. Temporal patterns present in the response of a population of fibers encoded the parameters of resonant stimuli. 6. The model revcor filter bank provided a means of studying temporal response patterns of the population of fibers to other complex sounds. 7. The output of the population model is a representation of the temporal information provided by the auditory periphery to the central nervous system; thus it provides a potentially useful tool for testing hypotheses concerning the processing of temporal information by the central auditory system.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1987

Effects of interaural time delays of noise stimuli on low-frequency cells in the cat's inferior colliculus. III. Evidence for cross-correlation.

1. We tested the coincidence, or cross-correlation, model of Jeffress, which proposes a neuronal mechanism for sensitivity to interaural time differences (ITDs) in low-frequency cells in the central nucleus of the inferior colliculus (ICC) of the cat. Different tokens of Gaussian noise stimuli were delivered to the two ears. We studied the neural responses to changes in ITDs of these stimuli and examined the manner in which the binaural cells responded to them. All of our results support the idea that the central binaural neurons perform an operation very similar to cross-correlation on the inputs arriving from each side. These inputs are transformed from the actual acoustic signal by the peripheral auditory system, and these transformations are reflected in the properties of the cross-correlations. 2. The responses to ITDs of identical broadband noise stimuli to the two ears varies cyclically as a function of ITD at a frequency close to the best frequency of the neuron. This cyclic response is a consequence of the narrowband filtering of the wideband acoustic signal by the auditory nerve fibers. To examine the effects of using stimuli to the two ears that were correlated to each other to different degrees, we generated pairs of noises. Each pair consisted of one standard noise, which was delivered to one ear, and a linear sum of two standard uncorrelated noises, which was delivered to the other ear. The responses of 34 neurons in the ICC to ITDs of noises with variable interaural coherence were examined. When partially correlated noises were delivered, there was a positive and approximately linear relationship between the degree of modulation of the response as a function of ITD and interaural coherence. The degree of modulation was measured by the synchronization coefficient, or vector strength, over one period of the ITD curve. 3. We examined the effects of altering the interaural phase relationships of the input noise stimuli. The phase of the noise stimuli was changed by digitally filtering the standard noise so that only a phase delay was imposed. The responses to ITDs with differing interaural phase relationships were then studied by delivering a phase-shifted noise to one ear and the standard noise to the other. The ITD curves in response to phase-shifted noise were shifted by about the same amount as the shift of the stimulus; the shift of the response was measured with respect to the case with identical noises to the two ears.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1986

A temporal analysis of auditory-nerve fiber responses to spoken stop consonant-vowel syllables.

Auditory-nerve fiber spike trains were recorded in response to spoken English stop consonant-vowel syllables, both voiced (/b,d,g/) and unvoiced (/p,t,k/), in the initial position of syllables with the vowels /i,a,u/. Temporal properties of the neural responses and stimulus spectra are displayed in a spectrographic format. The responses were categorized in terms of the fibers' characteristic frequencies (CF) and spontaneous rates (SR). High-CF, high-SR fibers generally synchronize to formants throughout the syllables. High-CF, low/medium-SR fibers may also synchronize to formants; however, during the voicing, there may be sufficient low-frequency energy present to suppress a fiber's synchronized response to a formant near its CF. Low-CF fibers, from both SR groups, synchronize to energy associated with voicing. Several proposed acoustic correlates to perceptual features of stop consonant-vowel syllables, including the initial spectrum, formant transitions, and voice-onset time, are represented in the temporal properties of auditory-nerve fiber responses. Nonlinear suppression affects the temporal features of the responses, particularly those of low/medium-spontaneous-rate fibers.

Acoustic Stimulation↗

PubMedDate not supplied

Nonlinear feedback models for the tuning of auditory nerve fibers.

The tuning of auditory nerve (AN) fibers is generally characterized by an increase in bandwidth and, for mid- to high-frequency fibers, a downward shift in the center frequency as sound level increases. Changes in bandwidth are accompanied by changes in the phase properties of the fibers; thus the timing of neural discharges also changes as a function of sound level. This study focuses on the magnitude and phase properties of models designed to reproduce the nonlinear properties of AN fibers that were studied electrophysiologically. The forward path of each model consisted of a linear second-order resonance, and each feedback path contained a saturating nonlinearity. In model 1, the feedback path was a simple memoryless, saturating nonlinearity. In model 2, a low-pass filter was added after the feedback nonlinearity. The ability of each model to simulate aspects of the nonlinear tuning of AN fibers is discussed. Model 2 was able to simulate a wider range of nonlinear behavior for different AN fibers and thus has promise for use in simulations of populations of fibers tuned to different frequencies.

Animals↗